a=[[1, -2, 3, -4], [5, -6, 7, -8], [9, 0, 1, -2]]
# a[0][1]=0
# a[0][3]=0
# a[1][1]=0

for i in range(len(a)):
    for j in range(len(a[i])):
        if a[i][j]<0:
            a[i][j]=0
print(a)